1. Field of the Invention
The present invention relates to full-type rolling bearings and roller cam followers for engines that have no cage, such as bearings for rocker arms, cam followers and roller follower bearings.
2. Description of the Background Art
Of recent rolling bearings, full-type roller bearings without cage like bearings for rocker arms to be used for high-speed heavy-load applications are increasing in number. In the full-type roller bearing without cage, it inevitably occurs that rollers interfere with each other. Therefore, at high speeds, the rollers are not properly controlled in terms of their positions so that skew is likely to occur. Resultant heat generation due to sliding as well as a local increase in surface pressure are likely to cause surface damages (peeling, smearing, surface-initiated peeling) and inside-initiated peeling, while the full-type roller bearing should have a high load capacity according to calculation.
More specifically, in such full-type roller bearings as roller follower, cam follower and rocker arm, the interference between rollers and poor supply of lubricant into the bearings could cause peelings initiated from the surfaces of rollers and raceways. Moreover, influences of an assembly error and a biasing load could cause skew of rollers, resulting in surface-initiated peelings due to sliding and inside-initiated peelings due to a local increase in surface pressure. The full-type roller bearing herein refers to a bearing without cage as described above and may sometimes be abbreviated as full roller bearing.
For a roller cam follower of an engine that has an outer ring with its circumference being in rolling contact with a cam, most improvements of the roller cam follower have been for the purpose of improving the circumference of the outer ring. For example, compression residual stress induced by such a process as shot peening and increased hardness induced by high-concentration carbonitriding (process-induced effects) have been used for prolonging the lifetime, chiefly for improving the circumference of the outer ring that is in rolling contact with the cam.
Although improvements with the aim of prolonging the rolling life of a roller shaft serving as an inner ring, rollers and the entire bearing have been relatively few, some improvements have still been made in terms of materials for providing heat resistance and microstructure stability as well as increased hardness that are obtained through carbonitriding, and thereby prolonging the lifetime of the bearing. There have been known techniques regarding prolonging of the lifetime of the roller cam follower for the engine:
(d1) For a cam follower bearing of an engine valve mechanism, a calculated life of the bearing, at a rated engine rpm, of 1000 hours or longer is achieved (Japanese Patent Laying-Open No. 2000-38907).
(d2) In order to achieve a bearing shaft of a cam follower with the properties of: a carbide ratio=10–25%; ratio of decomposed austenite content to initial retained austenite content=1/10–3/10; end hardness=HV 830–960; and average wavelength of surface roughness=25 μm or less, a bearing steel is carbonitrided and hard shot peened (Japanese Patent Laying-Open No. 10-47334).
(d3) A solid lubricant film of high polymer for example is formed on a cam follower shaft for improving wear resistance of the shaft (Japanese Patent Laying-Qpen No. 10-103339).
(d4) A cam follower shaft is made of a tool steel for example and is ion-nitrided or ion-plated at a temperature lower than a tempering temperature so as to have a high hardness (Japanese Patent Laying-Open No. 10-110720).
(d5) A cam follower bearing for an engine valve mechanism that has its shaft for which a bending stress is 150 MPa or less (Japanese Patent Laying-Open No. 2000-38906).
(d6) A cam follower for an engine valve mechanism that has a phosphate film which is excellent in lubricating-oil retention and provided on a rolling surface of a bearing component (Japanese Patent Laying-Open No. 2002-31212).
(d7) A cam follower for an engine valve mechanism that has a crowning in a region of a shaft where rollers roll (Japanese Utility-Model Laying-Open No. 63-185917).
(d8) A carburized shaft has a rolling surface layer which is high-concentration carburized or carbonitrided with a carbon concentration of 1.2%–1.7% and has an internal hardness of HV 300 (Japanese Patent Laying-Open No. 2002-194438).
There is another problem regarding the rocker arm as described below. In such a case that both ends of a roller shaft are caulked to be fixed to a roller supporting member, although a rolling surface of the roller shaft should have high hardness, the ends thereof should be soft enough to be caulked. Further, after the shaft ends are caulked to be fixed, the strength (hardness) should be high for preventing loosening in use. The following document discloses caulking of both ends of a roller shaft of a roller rocker.
(d9) The outer surface of a roller shaft is uniformly high-frequency induction-hardened and then tempered, and thereafter only the ends of the shaft are high-frequency annealed and accordingly softened (Japanese Patent Laying-Open No. 5-179350).
It is assumed that, the full-type roller bearings like the rocker arm, roller follower and cam follower will, similar to normal caged bearings, increase in speed and load in use, and the viscosity of a lubricating oil therefor will decrease. In order to extend the rolling life of the full-type roller bearings under such conditions in use, (a1) any measure should be taken, as usually done, for the rolling fatigue life dependent on the load and (a2) any measure should further be taken for the surface damage life due to metal contact caused by sliding and loss of an oil film. However, there has been no technique for remarkably extending both of the rolling fatigue life dependent on the load and the surface damage life due to the metal contact. Moreover, in addition to these two measures for prolonging the life, (a3) any measure should be taken for the issue of shortening of the life due to the interference of rollers with each other as well as the skew thereof that are peculiar to the full-type roller bearings.
The above-described known techniques improve the rolling life by increasing the hardness and the compression residual stress, or improve the rolling surface where a bearing component is in rolling contact with a counterpart component. In actually evaluating these techniques, it is found that they are effective for extending the life in such an application where bending is applied as in the case of the outer ring, while such improvements are not necessarily effective by themselves for extending the life of the inner ring and rollers of the full roller bearing.